Mapping Center Answer:

I was able to reproduce what you were seeing, and used a repeat labels setting for Maplex of 4 inches.  In order to get highway shields to work properly, you will also need a complementary Remove Duplicates setting that is just under 4 inches (I used 3.5 inches and it worked well).  Also don't do like I did the first time and forget to change the units to inches (from the default of map units).

So that should resolve the frequent placement issue.  That said, the use of 4 inches as a general spacing is something that will vary with map scale, and to some extent with respect to the number of characters in the label for the interstate or road shield.  Basically the fewer the characters, the longer the segments tend to be and therefore the need for repetition is less.  But for routes that are generally shorter, participating in complex closely spaced interchanges, it is important to label each segment of the route at least once, so you'll want a shorter repeat labels and remove duplicates setting.   The way to do that is to create two label classes, where the differentiator is SQL queries that use SQL CHAR_LENGTH function, and these would look like:  

"Type" = 'Interstate' AND CHAR_LENGTH( "LabelField") > 2

"Type" = 'Interstate' AND CHAR_LENGTH( "LabelField") <= 2

This will allow you to independently apply the distances.  The CHAR_LENGTH 

Last, a little bit about the Street Placement setting in Maplex:  It was designed to facilitate labeling surface streets and to be intelligent about contiguity and laddering street names along adjacent parallel sets of streets, so they look nice.  We didn't, therefore name it "Road Placement", leading people to expect it to do special things with route labels like we are discussing.  So, you were on the right track, and this should be what you need to finish.